This invention relates to an apparatus and method for anchoring a bolt in a rock formation, and, more particularly, to a tensioning assembly adapted for use with a bonding material.
It is well-known in the art of mine roof support to utilize systems which combine mechanical anchoring and resin anchoring. Typically, the system has an expansion shell assembly threadedly engaged to the end of a bolt which includes a roof support plate. A camming plug, used in combination with an expansion shell, usually employs a displaceable stop means to prevent axial movement of the bolt beyond a certain point in the plug when the effective torque applied to the bolt is below a certain level. Resin components are then inserted into the bore and the entire roof support assembly is placed immediately beneath them. Mixing of these components is accomplished by rotating the bolt, thereby causing the shell assembly similarly to rotate.
As the resin hardens, rotation of the shell assembly is resisted, thereby allowing the torque applied by the bolt to the displaceable stop means to exceed a predetermined level. This action displaces the stop means and permits relative rotational movement between the shell assembly and the bolt. Upon continued rotation of the bolt in the same direction, the plug advances axially along the bolt, expanding the shell into engagement with the bore hole. Once the resin is hardened, the bolt can be tensioned in the bore hole.
One of the major shortfalls of these systems lies in the construction of the stop means. The most familiar stop means is in the form of a shearable pin which breaks upon application of a preselected torque. Severe problems exist in practice, however, in determining the proper material and associated pin dimensions for constructing a pin which reacts to stress as planned. As a result, there has been little consistency in the operation of these systems, with many systems failing to operate properly in actual use.
Therefore, the principal object of the present invention is to provide a method and apparatus for combining resin bonding and mechanical anchoring of a bolt in a rock formation by a tensioning assembly provided with a resistance means which will thread in place in the assembly when a torque in excess of a predetermined value is applied to a bolt abutting the resistance means.
Another object is to provide a resistance means which is tapped by a bolt upon application of a torque in excess of a predetermined torque.